The attack of tricalcium phosphate by acids, particularly nitric, sulfuric and phosphoric, adapted to product phosphate salts soluble in neutral ammonium citrate and if desired in water, have been for a long time used industrially in the production of fertilizers and phosphate products for animal feed.
Urea is also employed, on a large scale, as nitrogenous fertilizer. Because of the major position it occupies in most of the developing countries, it constitutes even today the world's principal source of nitrogen used for fertilization of soil. Urea also has uses in animal feed.
It has on the other hand been known for a long time (the first published papers on the subject dating from 1935) that sulfuric acid combines with urea and that the mono- and bi-urea sulfates obtained have the property of forming eutectics. Processes using compositions of this type, adapted to produce liquid nitrogenous fertilizers (U.S. Pat. No. 4,116,664 of Sep. 26, 1978 and U.S. Pat. No. 4,310,343 of Jan. 12, 1982) or to treat cellulosic materials (U.S. Pat. No. 4,818,269 of Apr. 4, 1989) have been recently described.
Compositions based on phosphate salts and urea are also known and used in the production of complex granulated fertilizers. They are generally obtained by mixture of urea and phosphated compounds of the superphosphate type, produced by the attack of natural phosphate with sulfuric and/or phosphoric acid. But the industrial production and use of these products is attended by numerous difficulties which have until now greatly limited their development.
These difficulties arise from the fact that urea, which is always found in these compositions essentially in the free state, has a strong tendency to decompose above 60.degree. C. (that is below the temperatures which are usually used to dry the fertilizers) liberating ammonia which causes the reversion of monocalcium phosphate to bicalcium phosphate. It has, on the other hand, the property of substituting itself for water in the hydrates of calcium sulfate and/or mono- and bicalcium phosphates present in superphosphates, liberating free water which promotes the deliquescence of the granules of fertilizer and their hardening into a mass.
Also the products produced by these processes are extremely difficult to dry. Their shelf lives are on the other hand very mediocre, which practically excludes the possibility of distributing them in bulk.
The document PI 7908335 describes a process for the production of phospho-nitrogenous products whose P.sub.2 O.sub.5 is substantially entirely soluble in neutral ammonium citrate and in which there remains no significant quantity of free urea, this latter being, for the most part, combined in a urea compound of calcium sulfate, in which tricalcium phosphate is reacted with a reagent obtained by mixing sulfuric acid and urea containing a quantity of water less than 10% by weight and preferably of the order of 5%.
There is thus produced a double decomposition reaction, in the course of which:
the tricalcium phosphate is substantially transformed (to the extent of 80 to 90%) into monocalcium and urea phosphates, soluble in water and neutral ammonium citrate, and as to a substantially smaller portion (of the order of 5 to 15%) into bicalcium phosphate, perhaps combined with molecules of water and urea, soluble in neutral ammonium citrate; PA1 the sulfate ions combine with a portion of the calcium ions and the urea to form tetra-urea calcium sulfate of the formula: CaSO.sub.4.4CO(NH.sub.2).sub.2. PA1 E.sub.1 : 1.3 mole of urea per mole of sulfuric acid; PA1 E.sub.3 : 3.6 moles of urea per mole of sulfuric acid. PA1 from the calcium content of the natural phosphate can be evaluated the number M.sub.1 of moles of Ca.sup.2+ contained in 1 kg of phosphate; PA1 from the P.sub.2 O.sub.5 content of the same phosphate, can be evaluated the number M.sub.2 of moles of P.sub.2 O.sub.5 contained in 1 kg of the phosphate; PA1 the number M.sub.3 of moles of Ca.sup.2+ available for combination with the SO.sub.4.sup.2- ions is equal to M.sub.1 -M.sub.2 (assuming the simplified hypothesis that all the P.sub.2 O.sub.5 in the final product is combined with calcium ions in the form of monocalcium phosphate); PA1 the number of moles of SO.sub.4.sup.2- applied by the sulfo-ureic reagent must therefore be equal at least to M.sub.3.
Substantially all the P.sub.2 O.sub.5 contained in the final product is soluble in neutral ammonium citrate and a large proportion of this P.sub.2 O.sub.5, generally greater than 80%, is soluble in water.
In the presence of water, the ureates hydrolyze freeing the urea they contain. Also, when there are applied to the products according to the invention the analytical methods normally used to control fertilizers (these latter always being subjected first to solution of the product in water), it is seen that substantially all of the nitrogen that they contain is urea nitrogen.
But the combination of the urea in the form of ureates confers on the products obtained physico-chemical characteristics very different from those of compounds produced by known methods, in which the major portion of the urea is present in the free state. Thus, they possess, without the need to dry them, excellent properties for storage in bulk. They are thus less hygroscopic and they have no tendency to deliquesce.
The document PI 7908335 also proposes to react calcium phosphate with a preferably liquid mixture comprising at least one mole of urea per mole of sulfuric acid.
However, the urea salts described in this document and in particular the sulfates, are salts crystallized at ambient temperature whose use requires working at higher temperatures and the introduction of relatively great quantities of water.
Moreover, the products obtained according to document PI 7908335 must be dried by treatments which are not without effect on the quality of the final product.